Fix Critical Round-Trip Conversion Issues

[kamil-kaszuba]

🐛 Fix Critical Round-Trip Conversion Issues

📋 Summary

This PR fixes critical bugs in string case conversion methods that were causing round-trip conversions to fail, along with a modernized test suite migration to xUnit.

🚨 Critical Bug Fixed

Round-Trip Conversion Failures

Issue: Converting between different string cases and back to the original format was producing incorrect results.

Example of the main failure:

// Before (BROKEN):
"example-variable-name-321-test-a".ToPascalCase()
// Result: "Examplevariablename321testa" ❌

"ExampleVariableName321TestA".ToKebabCase().ToPascalCase()  
// Expected: "ExampleVariableName321TestA"
// Actual:   "Examplevariablename321testa" ❌

// After (FIXED):
"ExampleVariableName321TestA".ToKebabCase().ToPascalCase()
// Result:   "ExampleVariableName321TestA" ✅

Root Cause & Fix:

Problem: Word boundary detection failed when transitioning from special characters (dashes/underscores) back to letters, breaking the separator insertion logic.

Solution: Enhanced separator logic to detect transitions from special characters:

// Added condition to detect word boundaries after separators
insertSeparator = (previous != current && (current is UppercaseLetter || current is DecimalDigitNumber)) 
      || (IsSpecialCharacter(previous) && !IsSpecialCharacter(current)) // ← This was missing
      || insertSeparator;

✅ Impact

• 36 failing tests → All 88 tests passing ✅
• Round-trip conversions now work reliably across all case formats

🧪 Test Suite Modernization

Side improvement: Migrated from MSTest to xUnit with data-driven testing

• Reduced from 88 individual test methods to 4 consolidated methods

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Type: 🐛 Critical Bug Fix
Breaking Changes: None
Validates: All round-trip conversions now preserve original values

[kamil-kaszuba]

@SzymonHalucha will you take a look?